Watch Out For White Mold

When the extension agent asked for volunteers to plant varietal plots for white mold comparisons, David Hand was the first to step forward.

In 1997, parts of one field on his farm near Antwerp, OH, had a 60-70% yield reduction due to white mold.

"In areas of the field, we had more sclerotia coming into the tank than beans," Hand says about the harvest season.

The small black sclerotia develop on or inside infected stems. And a white cottony substance develops on the external stem tissue if conditions are cool and wet at flowering.

White mold ranks as one of the most devastating soybean diseases. Infestations can commonly cut yields 10-20% and, in extreme cases, up to 50-70%. And the pathogen can survive 10-12 years in the soil.

White mold has been a problem in soybeans since the 1970s in Michigan, Wisconsin and Minnesota. It wasn't until about 1992 that it started to be noticed in other Midwestern states.

"The disease appeared almost simultaneously from Ohio to Nebraska," points out Craig Grau, University of Wisconsin extension plant pathologist. "The reason for the outburst was cooler, wetter conditions in the '90s, a low level of fungus in seed production and high-yield production techniques."

Strategies that favor high yields also spur white mold development. Early planting, narrow rows, high seeding rates and good soil fertility are ideal environments for the disease. A quick-closing canopy fosters the cool, moist environment.

Like other growers, Hand finds the disease hits the most productive soils. Low-lying areas surrounded by trees are also more prone to problems, he says.

Hand, who farms with his wife, Deb, searches for the most resistant varieties to fight the mold. No varieties have total resistance, however.

In 1998, he planted 30 varieties replicated four times. Anne Dorrance, Ohio State University plant pathologist, conducted the research.

"In David's field there were five sclerotia per square foot, which is an incredible number," Dorrance says.

However, disease pressure was minimal due to 90 degree temperatures and dry weather in August. Cooler temperatures and 14-16 hours of wetness during flowering are needed for the white mold to develop.

Research results can be slow because the disease doesn't hit every year, and varietal response differs from one region to another. For example, Hand planted three varieties on his production acreage in '98.

"Only one was very good; the others were relatively poor. All were supposed to have good tolerance," Hand says.

In 1999, he planted half his acreage using the best-rated variety. Even so, the seed company's literature showed this same variety was not as tolerant in the most recent tests.

In 1996, scientists in the North Central Region began a white mold project. The states include Illinois, Indiana, Iowa, Michigan, Minnesota, Nebraska, North Dakota, Ohio, South Dakota and Wisconsin.

"We wanted to speed up the process of evaluating varieties, find out what effect production techniques had and find out if the information in one state is relevant in other states," Grau points out.

The researchers find that earlier-maturing varieties tend to be less susceptible to white mold than later-maturing varieties in certain regions. However, says Grau, partially resistant varieties can be found in the later-maturity group and some of the most susceptible varieties can be found in the early maturity group.

Be sure to check state soybean trials and seed company tests for your region, scientists say.

Research also uncovered that varieties with Williams background are more susceptible to white mold.

"When we started using the high-yielding, big, tall bushy soybeans, we got into trouble," Dorrance says.

Researchers across the continent are diligently searching for improved resistance. Several projects show promise.

Ohio scientists John Finer and Ron Fioritto introduced a wheat gene into soybeans. The gene breaks down oxalic acid, which is essential for white mold development. Researchers at Wooster, OH, are testing the soybeans in the field for the first time in 1999.

The same gene is being tested in Nebraska, Illinois and Ontario, and Michigan workers are using a gene from barley.

"With this many people working on the research, it shows definite promise of being an effective tool," says Dorrance.

However, don't expect to see the soybeans available next year. Wisconsin's Grau estimates that these highly resistant white mold soybeans won't be in farmers' hands for five years or longer.

Until they hit the market, growers are combining the most-tolerant varieties with other methods.

Hand also suggests reducing plant populations as another strategy to avoid white mold. He varies seeding rate by field from 205,000 to 175,000 seeds per acre.

Another approach is to apply fungicides, though expense generally limits this method to seed producers. Benlate and Topsin M are registered for soybeans. They're effective, but must penetrate the canopy and cover flowers on the lower nodes.

A newer treatment on the market is Cobra herbicide. A 4- to 6-oz/acre rate is applied when soybeans are blooming. Cobra has suppressed white mold in moderately susceptible varieties in some tests. It should be considered only for high-risk fields, however. Iowa State University research showed applying Cobra when white mold wasn't present hurt yields.

Iowa State plant pathologists have taken another approach. Charlie Martinson, X.B. Yang and Luis del Rio are using a parasitic fungus that attacks the sclerotia in the soil.